ANDROID: kbuild: speed up ksym_dep_filter

This commit reduces the input that tr and sed need to consider to a
few lines and so makes the extraction of ksyms faster.

On my machine at least, builds are greatly slowed down by tr.

A single invocation of sed might be fast too but seems non-trivial.

Bug: 174773617
Change-Id: I98698ff8603d816c55d4922ccb5936364b5fd906
Signed-off-by: Giuliano Procida <gprocida@google.com>